1. Field of the Invention
The present invention generally relates to a disk drive, and more particularly to a disk drive including a mechanism for sustaining a feedback filter state having a periodic output.
2. Description of the Related Art
At high track density (e.g., currently 15,000 tracks per inch (TPI), but expected to grow to 25,000 TPI by year 2000) track-following mode, a disk drive is ultra-sensitive to disturbance sources acting on it. Some disturbance sources may contain components at unique frequencies dictated by the physical processes associated with them. Among the sources, vibration and disk flutter are two major processes that contribute to high track density limitation.
A disk drive with a sector servo architecture requires a fine position error signal (PES) in track-following mode for accurate positioning of the head. The fine PES is capable of representing the relative head-to-track motion as a percentage of track pitch with better than 8% (3-sigma) error. During a seek, only the Gray code that represents the track number is mandatory. The fine PES captures the relative motion between a track and a head not accurately measured at high seek velocity.
Hence, during a seek, a reliable fine PES is not available, and the actuator control current is two orders of magnitude higher. Moreover, all components of the track-following servo algorithm are disconnected and deactivated during a seek. It has been demonstrated that, by employing a gain enhancing digital filter tuned to a given disturbance frequency, the track-following error of a disk drive can be reduced.
According to a conventional system and method, the digital filter state must be determined (stored) first, and then the initial condition of the filter must be set accordingly while connecting the filter during a track-following mode. Thus, the settle-out time penalty that would normally occur due to the xe2x80x9clearningxe2x80x9d time required for a feedback filter is minimized. When a single filter is employed against a disk-shift problem or xe2x80x9cspindle-induced vibrationxe2x80x9d, the conventional method works effectively.
However, when multiple filters are used to compensate for multiple frequency-based disturbance sources, the conventional method becomes limited in three areas as follows.
That is, the memory required to store the respective filter states increases proportionally to the number of filters used. Further, when the disturbance process is unsteady or not coupled to the spindle rotational position of the drive of interest, the phase drift encountered by a filter state may render the xe2x80x9cstoredxe2x80x9d filter state inaccurate for filter initialization, thereby requiring a frequent filter state updating process. Additionally, when the frequency of the disturbance is not an exact an integer multiple (or at least substantially close to an integer multiple, such as by about 1-4% depending on the number of disk revolutions) of the spindle fundamental frequency, the addressing scheme required to extract the initial condition at the end of a seek becomes complex.
Therefore, the conventional systems and methods require large memory, complex addressing, and frequent initial condition updates.
Another problem of the conventional system is disk warpage (e.g., waviness) which causes the initial condition of the filter state not to be reliable/accurate. That is, an error component generated due to disk warpage (e.g., caused by clamping of the disk or the like) may differ dependent upon the radial position of the disk. The conventional systems and methods cannot compensate for such a problem.
In view of the foregoing and other problems of the conventional methods and structures, an object of the present invention is to provide a method and structure in which memory requirements are minimized, addressing requirements are simple, and frequent initial condition updating are avoided.
According to a first aspect of the invention, a disk drive system includes a head, a controller for controlling an operation of the head, at least one feedback filter coupled in relation to the controller, and a mechanism for keeping a function of the at least one feedback filter active during any of an absence of a valid position error signal (PES) and a position error signal being unavailable due to a defect during an operation of the head.
The unique and unobvious structure and method of the present invention solves the problem of sustaining the track-following feedback filter state in the absence of an accurate fine position error signal (PES) encountered during a seek (or unavailability of the PES due to a defect such as a magnetic defect) so that a special filter initialization process is eliminated. By keeping the feedback filter functions active during the absence of a fine PES, as is the case during a seek, the invention eliminates the use of stored filter state required for each filter employed.
The present invention provides novel approaches to solve the problem of feedback filter utilization when seek and track-following operations are repeatedly executed.
Several methods have been tested on experimental hardware and shown to effective and to provide superior results. A first method simply allows the feedback filter(s) to continue to coast along as if the filters(s) are still in the track-following mode by providing a Null-PES input during a seek. The state of the same filter continues to evolve based on the initial state value left over just prior to a seek. At the end of a seek, the Null-PES input is replaced by the newly measured PES stream.
An enhancement to the first approach is to replace the nominal feedback-filter coefficients by a filter with identical peak-frequency, but with zero damping so that the state evolution is sustained without any dissipation of the filter sinusoidal amplitude while preserving the amplitude during the seek mode.
A second method of the present invention employs a dedicated PES memory buffer, from which a pseudo-PES stream is extracted and input to the filter that would sustain the filter state during a seek. At the end of a seek, the filter input is replaced by the newly measured PES stream. The PES-buffer is persistently upgraded while in a track-follow mode by the most recent PES stream, as indexed by the sector number regardless of the track number. With the second method, the most recent disturbance characteristics are retained in the PES-buffer.
When the frequency of disturbance is an integer multiple (or substantially close thereto) of the rotating speed of the spindle of the product of interest, the length of the PES buffer is exactly equal to the total number of servo sectors in a track. However, if the disturbance frequency is lower than that of the spindle speed, the PES-buffer length is extended to cover a complete cycle of the slowest frequency of interest. Individualized address pointers are allocated for each disturbance component that corresponds to a non-integer multiple of the spindle rotating speed. These address pointers are offset by a number that is computed to be a function of its period.
Depending on the nature of the disturbance, a combination of the first and second approaches can be optimally used.
Thus, the method and structure of the present invention minimizes memory requirements, uses simple addressing, and avoids frequent initial condition updates.